09 枚举类

枚举概述

枚举用来管理一组相关常量的集合，使用枚举可以提高程序的可读性，使代码更清晰且更易于维护。

在 Java 5 之前没有提供枚举类型，可以通过声明静态常量（final static 变量）替代枚举常量。
通常在接口中声明一组静态常量，当然也可以在一般类中声明一组静态常量。这些常量往往都是 int 类型，这是为了以后方便使用 switch 语句进行判断。

public interface WeekDays {
    // 枚举常量列表
    int MONDAY = 0; // 星期一
    int TUESDAY = 1; // 星期二
    int WEDNESDAY = 2; // 星期三
    ... ...
}

使用常量的缺点：

  1. 类型不安全。程序执行过程中很有可能给 day 变量传入一个任意的整数值，可能导致程序出现错误。
  2. 程序不方便调试。在程序调试时，如果通过日志输出day值，那么只能看到 0~4 之间的数值。

枚举类型可以避免直接使用常量所导致的问题。Java 5 之后可以使用枚举类型了，Java中枚举类型的作用已经不仅仅是定义一组常量提高程序的可读性了，还具有如下特性：

  1. Java 枚举类型是一种类，是引用类型，具有了面向对象特性，可以添加方法和成员变量等；
  2. Java 枚举类型继承 java.lang.Enum，且不需要显式声明；
  3. Java 枚举类型可以实现接口，与类实现接口类似；
  4. Java 枚举类型不能被继承，不存在子类；
  5. 枚举类型都实现了Java API中的 Comparable 接口，都可以通过方法compareTo与其他枚举值进行比较。比较其实就是比较 ordinal 的大小；
  6. 可搭配 switch 语句使用，在 switch 语句内部，枚举值不能带枚举类型前缀。

枚举类声明

[public] enum 枚举名 {
     枚举常量列表
}

“枚举名”是该枚举类的名称。它首先应该是有效的标识符，其次应该遵守 Java 命名规范。建议以 Enum 结尾。

枚举类中成员变量和成员方法

枚举类可以像类一样包含成员变量和成员方法，成员变量可以是实例变量也可以是静态变量，成员方法可以是实例方法，也可以是静态方法，但不能是抽象方法。

枚举类构造方法

    public enum WeekDayEnum {
        // 枚举常量列表
        MONDAY("星期一", 0), TUESDAY("星期二", 1), WEDNESDAY("星期三", 2), THURSDAY(
                "星期四", 3), FRIDAY("星期五", 4);

        // 实例变量
        private final String name;
        private final int index;

        private WeekDayEnum(String name, int index) {
            this.name = name;
            this.index = index;
        }

    }

注意枚举类的中的构造方法只能是私有访问级别，构造方法可以省略 private 关键字，但它仍然是私有的构造方法。这也说明了枚举类不允许在外部创建对象。

枚举常用方法

所有枚举类都继承java.lang.Enum类，Enum中定义了一些枚举中常用的方法：

  • int ordinal()：返回枚举常量的顺序。每个枚举值经常有一个关联的标识符，为枚举常量声明的顺序，ordinal值会从从数字 0 开始。
  • 枚举类型[] values()：静态方法，返回一个包含全部枚举常量的数组。
  • 枚举类型 valueOf(String str)：静态方法，str 是枚举常量对应的字符串，返回一个包含枚举类型实例。

jdk1.8 Enum 的源码

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
    /**
     * The name of this enum constant, as declared in the enum declaration.
     * Most programmers should use the {@link #toString} method rather than
     * accessing this field.
     */
    private final String name;

    /**
     * Returns the name of this enum constant, exactly as declared in its
     * enum declaration.
     *
     * <b>Most programmers should use the {@link #toString} method in
     * preference to this one, as the toString method may return
     * a more user-friendly name.</b>  This method is designed primarily for
     * use in specialized situations where correctness depends on getting the
     * exact name, which will not vary from release to release.
     *
     * @return the name of this enum constant
     */
    public final String name() {
        return name;
    }

    /**
     * The ordinal of this enumeration constant (its position
     * in the enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     *
     * Most programmers will have no use for this field.  It is designed
     * for use by sophisticated enum-based data structures, such as
     * {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     */
    private final int ordinal;

    /**
     * Returns the ordinal of this enumeration constant (its position
     * in its enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     *
     * Most programmers will have no use for this method.  It is
     * designed for use by sophisticated enum-based data structures, such
     * as {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     *
     * @return the ordinal of this enumeration constant
     */
    public final int ordinal() {
        return ordinal;
    }

    /**
     * Sole constructor.  Programmers cannot invoke this constructor.
     * It is for use by code emitted by the compiler in response to
     * enum type declarations.
     *
     * @param name - The name of this enum constant, which is the identifier
     *               used to declare it.
     * @param ordinal - The ordinal of this enumeration constant (its position
     *         in the enum declaration, where the initial constant is assigned
     *         an ordinal of zero).
     */
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }

    /**
     * Returns the name of this enum constant, as contained in the
     * declaration.  This method may be overridden, though it typically
     * isn't necessary or desirable.  An enum type should override this
     * method when a more "programmer-friendly" string form exists.
     *
     * @return the name of this enum constant
     */
    public String toString() {
        return name;
    }

    /**
     * Returns true if the specified object is equal to this
     * enum constant.
     *
     * @param other the object to be compared for equality with this object.
     * @return  true if the specified object is equal to this
     *          enum constant.
     */
    public final boolean equals(Object other) {
        return this==other;
    }

    /**
     * Returns a hash code for this enum constant.
     *
     * @return a hash code for this enum constant.
     */
    public final int hashCode() {
        return super.hashCode();
    }

    /**
     * Throws CloneNotSupportedException.  This guarantees that enums
     * are never cloned, which is necessary to preserve their "singleton"
     * status.
     *
     * @return (never returns)
     */
    protected final Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    /**
     * Compares this enum with the specified object for order.  Returns a
     * negative integer, zero, or a positive integer as this object is less
     * than, equal to, or greater than the specified object.
     *
     * Enum constants are only comparable to other enum constants of the
     * same enum type.  The natural order implemented by this
     * method is the order in which the constants are declared.
     */
    public final int compareTo(E o) {
        Enum<?> other = (Enum<?>)o;
        Enum<E> self = this;
        if (self.getClass() != other.getClass() && // optimization
            self.getDeclaringClass() != other.getDeclaringClass())
            throw new ClassCastException();
        return self.ordinal - other.ordinal;
    }

    /**
     * Returns the Class object corresponding to this enum constant's
     * enum type.  Two enum constants e1 and  e2 are of the
     * same enum type if and only if
     *   e1.getDeclaringClass() == e2.getDeclaringClass().
     * (The value returned by this method may differ from the one returned
     * by the {@link Object#getClass} method for enum constants with
     * constant-specific class bodies.)
     *
     * @return the Class object corresponding to this enum constant's
     *     enum type
     */
    @SuppressWarnings("unchecked")
    public final Class<E> getDeclaringClass() {
        Class<?> clazz = getClass();
        Class<?> zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper;
    }

    /**
     * Returns the enum constant of the specified enum type with the
     * specified name.  The name must match exactly an identifier used
     * to declare an enum constant in this type.  (Extraneous whitespace
     * characters are not permitted.)
     *
     * <p>Note that for a particular enum type {@code T}, the
     * implicitly declared {@code public static T valueOf(String)}
     * method on that enum may be used instead of this method to map
     * from a name to the corresponding enum constant.  All the
     * constants of an enum type can be obtained by calling the
     * implicit {@code public static T[] values()} method of that
     * type.
     *
     * @param <T> The enum type whose constant is to be returned
     * @param enumType the {@code Class} object of the enum type from which
     *      to return a constant
     * @param name the name of the constant to return
     * @return the enum constant of the specified enum type with the
     *      specified name
     * @throws IllegalArgumentException if the specified enum type has
     *         no constant with the specified name, or the specified
     *         class object does not represent an enum type
     * @throws NullPointerException if {@code enumType} or {@code name}
     *         is null
     * @since 1.5
     */
    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumType.getCanonicalName() + "." + name);
    }

    /**
     * enum classes cannot have finalize methods.
     */
    protected final void finalize() { }

    /**
     * prevent default deserialization
     */
    private void readObject(ObjectInputStream in) throws IOException,
        ClassNotFoundException {
        throw new InvalidObjectException("can't deserialize enum");
    }

    private void readObjectNoData() throws ObjectStreamException {
        throw new InvalidObjectException("can't deserialize enum");
    }
}

参考

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